A few prior-art hot-air furnaces have employed two built-in blowers to vary the volume of air circulated for low and high heating phases, e.g. Kriechbaum U.S. Pat. No. 2,109,862 and Rifle U.S. Pat. No. 2,672,291. But in such furnaces usually both blowers and are high-powered, are not designed for attachment to existing furnaces.